The present invention concerns video image processing apparatus and in particular apparatus for converting image data between bit-plane and multi-bit pixel image representations.
Video data can be organized in a computer memory in a variety of formats. For each picture element (pixel) on the screen, there are usually several bits which together describe the properties of the pixel. These properties may be the color of the pixel, its brightness, or any of a number of other properties. The two most common formats for representing an image by digital values are bit-plane and multi-bit pixel. Each of these formats has advantages and disadvantages.
In the bit-plane format, a group of pixels is represented as a series of planes. Each plane contains one bit per pixel. Thus for an N-bit representation of image data, N planes of data are employed. The total memory storage needed to represent the image is the number of pixels in the image times the number of planes of data.
Bit-plane representation of the images is advantageous since it only uses the amount of data needed by the bit-planes which represent the image. For example, a monochrome image having no gray scale variations, such as may be used by a word processing program, can be represented by a single bit-plane. In addition, an image stored in bit-plane format may be modified by changing only a single plane or some set of planes without affecting the other planes. If, for example, the image includes foreground components on one set of bit-planes and background components on another set of bit-planes, the background bit-planes could be manipulated without affecting the foreground bit-planes.
Bit-plane image representation has its disadvantages as well. For example, to set a specific pixel to a specific color, all of the bits used to represent the pixel may need to be modified. This usually requires N different memory accesses for an image represented by N planes of data.
In the multi-bit pixel format each image pixel is typically represented by an eight bit byte. All bits in the byte are addressed by a single unique address value. Thus, for eight-bit pixel values, the amount of memory needed for any image is always eight-bits times the number of pixels in the image.
The multi-bit pixel format for representing images has many advantages. To set a specific pixel to a specific color, only a single memory location would need to be modified. Furthermore, in image processing it is often desirable to perform mathematical operations on the image pixels. These operations may be performed for such functions as spatial filtering, edge detection or pattern matching. The multi-bit pixel representation is a good format for these operations because each of the image pixels is represented by a number which may be conveniently manipulated by the computer.
The multi-bit pixel format has disadvantages as well. This format may require more memory than the bit-plane format to represent a given image. In addition, for certain types of image operations, such as the management of overlays, this format may require more data to be transferred and processed than if the equivalent operation were performed on a bit-plane representation of the image.